SANTA CLARA -- Adding to a bitter rivalry, the 49ers signed wide receiver Chris Harper off the Seahawks practice squad Monday, some two weeks before the 49ers visit Seattle in an NFC West battle.

Harper filled a 49ers roster spot created by Monday's trade of outside linebacker Cam Johnson to the Indianapolis Colts, reportedly for a 2014 seventh-round draft pick, according to NFL.com.

The Seahawks selected Harper in the fourth round of this year's draft, and at No. 123 overall, he came five spots ahead of 49ers rookie wideout Quinton Patton. Those two are part of a six-man receiving corps that doesn't include Michael Crabtree and Mario Manningham, who will start the season on the physically-unable-to-perform list.

Harper averaged 9.8 yards while catching 4 of 7 passes targeted for him in the Seahawks' exhibition season. He began his career at Oregon before transferring to Kansas State.

Johnson, a 2012 seventh-round pick, didn't last long as the fifth outside linebacker on this season's roster. His stock hit an all-time high Thursday when he had two sacks and blocked a punt that he recovered for a touchdown at San Diego.

"We appreciate Cam's efforts since joining our team a year ago," general manager Trent Baalke said in a statement. "He has continued to grow as a football player, and we wish him all the best with his new opportunity in Indianapolis."